Selection of fish has been constrained by the difficulties of communal raising due to the lack of a reliable tag for small fish. We have overcome this problem by the mixed rearing of the progeny of a complete 4 x 4 factorial cross of African catfish (Clarias gariepinus) in a single tank and microsatellite DNA fingerprinting a subsample at 1 month and 9 months of age. Simultaneously, these fish were scored for several phenotypic traits. Eighty five to 91% of all fish were identified uniquely on the basis of nine microsatellite DNA markers. The survival rare was affected by dam at 9 months of age. Dam and family effects were prominent in all phenotypic traits measured at 1 month of age, while sire and family effects were prominent at 9 months of age. Selection of the best growers on the basis of mass selection or family selection did not affect genotype probably because of the limited genetic background of the broodstock. Phenotypically, growth rate of the mass-selected group turned out to be higher than the family-selected group. (C) 1999 Elsevier Science B.V. All rights reserved.